Piezoelectric bulk layers with tilted c-axis orientation and methods for making the same

ABSTRACT

Methods for depositing bulk layer crystalline material having a predetermined c-axis tilt on a substrate include a first step of depositing a first portion of bulk layer material at a first incidence angle to achieve a predetermined c-axis tilt, and a second step of depositing a second portion of the bulk material layer onto the first portion at a second incidence angle that is smaller than the first incidence angle. The second portion has a second c-axis tilt that substantially aligns with the first c-axis tilt.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/646,213, filed 21 Mar. 2018 and U.S. Provisional Application No.62/646,208, filed 21 Mar. 2018, the disclosures of which areincorporated by reference herein in their entireties.

FIELD

The present disclosure relates to structures including inclined c-axishexagonal crystal structure materials, as well as systems and methodsfor producing such materials. In particular, the present disclosurerelates to structures including inclined c-axis hexagonal crystalstructure piezoelectric materials such as aluminum nitride (AlN) andzinc oxide (ZnO). Inclined c-axis hexagonal crystal structurepiezoelectric materials may be used, for example, in various resonatorsas well as in thin film electroacoustic and/or sensor devices,particularly for sensors operating in liquid/viscous media (e.g.,chemical and biochemical sensors), and the like.

BACKGROUND

Hexagonal crystal structure piezoelectric materials such as AlN and ZnOare of commercial interest due to their piezoelectric andelectroacoustic properties. A primary use of electroacoustic technologyhas been in the telecommunication field (e.g., for oscillators, filters,delay lines, etc.). More recently, there has been a growing interest inusing electroacoustic devices in high frequency sensing applications dueto the potential for high sensitivity, resolution, and reliability.However, it is not trivial to apply electroacoustic technology incertain sensor applications—particularly sensors operating in liquid orviscous media (e.g., chemical and biochemical sensors)—sincelongitudinal and surface waves exhibit considerable acoustic leakageinto such media, thereby resulting in reduced resolution.

In the case of a piezoelectric crystal resonator, an acoustic wave mayembody either a bulk acoustic wave (BAW) propagating through theinterior (or ‘bulk’) of a piezoelectric material, or a surface acousticwave (SAW) propagating on the surface of the piezoelectric material.

SAW devices involve transduction of acoustic waves (commonly includingtwo-dimensional Rayleigh waves) utilizing interdigital transducers alongthe surface of a piezoelectric material, with the waves being confinedto a penetration depth of about one wavelength. BAW devices typicallyinvolve transduction of an acoustic wave using electrodes arranged onopposing top and bottom surfaces of a piezoelectric material. In a BAWdevice, different vibration modes can propagate in the bulk material,including a longitudinal mode and two differently polarized shear modes,wherein the longitudinal and shear bulk modes propagate at differentvelocities. The longitudinal mode is characterized by compression andelongation in the direction of the propagation, whereas the shear modesconsist of motion perpendicular to the direction of propagation with nolocal change of volume. The propagation characteristics of these bulkmodes depend on the material properties and propagation directionrespective to the crystal axis orientations. Because shear waves exhibita very low penetration depth into a liquid, a device with pure orpredominant shear modes can operate in liquids without significantradiation losses (in contrast with longitudinal waves, which can beradiated in liquid and exhibit significant propagation losses).Restated, shear mode vibrations are beneficial for operation of acousticwave devices with fluids because shear waves do not impart significantenergy into fluids.

Certain piezoelectric thin films are capable of exciting bothlongitudinal and shear mode resonance. To excite a wave including ashear mode using a standard sandwiched electrode configuration device, apolarization axis in a piezoelectric thin film must generally benon-perpendicular to (e.g., tilted relative to) the film plane.Hexagonal crystal structure piezoelectric materials such as (but notlimited to) aluminum nitride (AlN) and zinc oxide (ZnO) tend to developtheir polarization axis (i.e., c-axis) perpendicular to the film plane,since the (0001) plane typically has the lowest surface density and isthermodynamically preferred. Certain high-temperature (e.g., vapor phaseepitaxy) processes may be used to grow tilted c-axis films, butproviding full compatibility with microelectronic structures such asmetal electrodes or traces requires a low temperature deposition process(e.g., typically below about 300° C.).

Low temperature deposition methods such as reactive radio frequencymagnetron sputtering have been used for preparing tilted AlN films.However, these processes tend to result in deposition angles that varysignificantly with position over the area of a substrate, which leads toa c-axis direction of the deposited piezoelectric material that varieswith radial position of the target to the source.

One effect of the lack of uniformity of c-axis tilt angle of the AlNfilm structure over the substrate is that if the AlN film-coveredsubstrate were to be diced into individual chips, the individual chipswould exhibit significant variation in c-axis tilt angle and concomitantvariation in acoustic wave propagation characteristics. Such variationin c-axis tilt angle would render it difficult to efficiently producelarge numbers of resonator chips with consistent and repeatableperformance.

Improved methods and systems for producing bulk films with c-axis tilthave been described, where the c-axis tilt of the bulk layer isprimarily controlled by controlling the deposition angle. For example, adevice and method for depositing seed and bulk layers with a tiltedc-axis are described in U.S. patent application Ser. No. 15/293,063entitled “Deposition System for Growth of Inclined C-Axis PiezoelectricMaterial Structures;” U.S. patent application Ser. No. 15/293,071entitled “Methods for Fabricating Acoustic Structure with InclinedC-Axis Piezoelectric Bulk and Crystalline Seed Layers;” U.S. patentapplication Ser. No. 15/293,082 entitled “Acoustic Resonator Structurewith Inclined C-Axis Piezoelectric Bulk and Crystalline Seed Layers;”U.S. patent application Ser. No. 15/293,091 entitled “Multi-StageDeposition System for Growth of Inclined C-Axis Piezoelectric MaterialStructures;” and U.S. patent application Ser. No. 15/293,108 entitled“Methods for Producing Piezoelectric Bulk and Crystalline Seed Layers ofDifferent C-Axis Orientation Distributions”. These published patentapplications also describe, among other things, the use of collimatorsand control mechanisms to provide for more uniform c-axis tilt of bulkfilms across the surface of the substrate. The use of collimators mayresult in a substantial amount of deposition of the bulk layer on thecollimator rather than on the substrate, which may lead to waste andinefficiencies in the process.

These published patent applications also describe attempts to deposit abulk layer directly onto a substrate without first depositing a seedlayer (see, for example, U.S. patent application Ser. No. 15/293,071).However, such bulk layers did not exhibit a desired c-axis tilt angledistribution and failed to exhibit a desired minimum shear mode tolongitudinal coupling ratio of at least 1.25 (which would result instructures that would not be suitable for use as bulk acoustic sensingresonators in liquid/viscous media), despite being deposited under thesame conditions as used for bulk layers deposited onto seed layers(which exhibited desired properties).

Further improvements are desired to provide, for example, one or moreof: additional control over the angle of the c-axis of crystals in bulklayers; improved characteristics such as mechanical quality factor,coupling coefficient, or shear to longitudinal coupling ratio; andimproved manufacturing efficiency of bulk layers.

SUMMARY

The present disclosure provides, among other things, bulk acoustic waveresonator structures and methods for fabricating such resonatorstructures. The bulk acoustic wave resonator structures include a bulklayer with inclined c-axis hexagonal crystal structure material (e.g.,piezoelectric material). The hexagonal crystal structure bulk layer issupported by a substrate. The bulk layer may be formed in a two-stepprocess. In the first step a portion of the bulk layer is deposited atan off-normal angle of incidence to achieve a desired c-axis tilt. Oncethe c-axis tilt is established, the remainder of the bulk layer isdeposited at normal incidence. Despite being deposited at normalincidence, the remaining bulk layer tends to adopt the c-axis tilt ofthe previously deposited crystal layer. Such processes may be performedwithout the use of a traditional seed layer, which tend to promote (103)texture with no in-plane alignment along the (002) direction, or suchprocesses may be performed using a traditional seed layer.

In some embodiments, a method for preparing a crystalline bulk layerhaving a c-axis tilt with a preselected angle includes deposition of afirst portion in a first growth step under deposition conditionscomprising a pressure of 5 mTorr or less. The first growth step isperformed at off-normal incidence. Preferably, the deposited bulk layerhas a c-axis tilt of about 35 degrees or greater. For example, the bulkmaterial layer may be deposited at a deposition angle of about 35degrees to about 85 degrees. Preferably, the deposition in the firstgrowth step is under conditions that retard surface mobility of thematerial being deposited such that crystals in the bulk material layerare substantially parallel to one another and are substantially orientedin a direction of the preselected angle. The method further comprisesdeposition of a second portion in a second growth step comprisingdepositing a bulk material layer at a smaller incidence angle, e.g.,about a normal incidence. Despite being deposited at about normalincidence, the second portion of bulk material layer deposited in thesecond growth step orients to the c-axis tilt of the first portion,e.g., about 35 degrees or greater. The bulk material may exhibit a ratioof shear coupling to longitudinal coupling of 1.25 or greater duringexcitation.

In some embodiments, a method comprises depositing a first portion of abulk material layer onto a substrate at a first incidence angle, thefirst portion having a first c-axis tilt; and depositing a secondportion of the bulk material layer onto the first portion at a secondincidence angle that is smaller than the first incidence angle, thesecond portion having a second c-axis tilt that substantially alignswith the first c-axis tilt. The first portion may be deposited directlyonto the surface of the substrate.

In some embodiments, a method comprises depositing a seed layer on asubstrate under deposition conditions comprising a pressure of 10 mTorror greater; and depositing a crystalline bulk layer having a c-axis tiltwith a preselected angle on the seed layer. Depositing the bulk layercomprises a two-step process. A first portion is deposited in a firstgrowth step performed at a first incidence angle that is an off-normalincidence angle. Preferably, the deposited first portion of the bulklayer has a c-axis tilt of the preselected angle. The method furthercomprises a second growth step comprising depositing a second portion ofbulk material layer at a second incidence angle that is smaller than thefirst incidence angle. Despite being deposited at about normalincidence, the second portion of bulk material layer deposited in thesecond growth step orients to the c-axis tilt of the first portion. Thebulk material may exhibit a ratio of shear coupling to longitudinalcoupling of 1.25 or greater during excitation.

In various embodiments described herein, the bulk layer is prepared suchthat the c-axis orientation of the crystals in the bulk layer isselectable within a range of about 0 degrees to about 90 degrees, suchas from about 30 degrees to about 52 degrees, or from about 35 degreesto about 46 degrees. The c-axis orientation distribution is preferablysubstantially uniform over the area of a large substrate (e.g., having adiameter in a range of at least about 50 mm, about 100 mm, or about 150mm), thereby enabling multiple chips to be derived from a singlesubstrate and having the same or similar acoustic wave propagationcharacteristics.

In various embodiments described herein, the bulk material layer has athickness of about 1,000 Angstroms to about 30,000 Angstroms. The bulkmaterial layer may be deposited at a deposition angle of about 35degrees to about 85 degrees. The bulk material may exhibit a ratio ofshear coupling to longitudinal coupling of 1.25 or greater duringexcitation.

In various embodiments described herein, a structure includes asubstrate comprising a wafer and a piezoelectric bulk material layerdeposited onto a surface of the wafer, where the bulk material layer hasa c-axis tilt of about 32 degrees or greater. The structure may exhibita ratio of shear coupling to longitudinal coupling of 1.25 or greaterduring excitation.

In various embodiments described herein, a bulk acoustic wave resonatorincludes a structure including a substrate comprising a wafer and apiezoelectric bulk material layer deposited onto a surface of the wafer,where the bulk material layer has a c-axis tilt of about 32 degrees orgreater, where at least a portion of piezoelectric bulk material layeris between the first electrode and the second electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plot of shear coupling coefficient (Ks) and longitudinalcoupling coefficient (Kl) as a function of c-axis angle of inclinationfor AN.

FIGS. 2A-2D are schematic views illustrating a process for depositing abulk layer on a substrate without a seed layer to achieve a desiredc-axis tilt in accordance with an embodiment described herein.

FIGS. 3A-3D are schematic views illustrating a process for depositing abulk layer on a substrate having a seed layer to achieve a desiredc-axis tilt in accordance with an embodiment described herein.

FIG. 4 is an upper exterior perspective view of a reactor of adeposition system for growing a hexagonal crystal structurepiezoelectric material with an inclined c-axis, the system including alinear sputtering apparatus, a movable substrate table for supportingmultiple substrates, and a collimator.

FIG. 5 is an upper perspective view of some of the elements of thereactor of FIG. 4, including a linear sputtering apparatus, atranslation track for translating a movable substrate table forsupporting multiple substrates, and a collimator.

FIG. 6 is a schematic cross-sectional view of a portion of a bulkacoustic wave solidly mounted resonator device including an inclinedc-axis hexagonal crystal structure piezoelectric material bulk layer asdisclosed herein, with the resonator device including an active regionwith a portion of the piezoelectric material arranged betweenoverlapping portions of a top side electrode and a bottom sideelectrode.

FIG. 7 is a schematic cross-sectional view of a film bulk acoustic waveresonator (FBAR) device according to one embodiment including aninclined c-axis hexagonal crystal structure piezoelectric material bulklayer arranged over a crystalline seed layer as disclosed herein, withthe FBAR device including a substrate defining a cavity covered by asupport layer, and including an active region registered with the cavitywith a portion of the piezoelectric material arranged betweenoverlapping portions of a top side electrode and a bottom sideelectrode.

FIGS. 8A and 8B are graphical representations of the c-axis angle ofsamples from an Example.

FIGS. 9A and 9B are graphical representations of the c-axis angle of acomparative sample from the Example.

FIG. 10 is a graphical representation of electromechanical coupling ofsamples and the comparative sample of the Example.

FIG. 11 is a graphical representation of wafer quality of samples andthe comparative sample of the Example.

DETAILED DESCRIPTION

The present disclosure relates to methods of depositing crystalline bulklayers that allow for selecting the c-axis tilt of the crystallinematerial. The bulk layer may be formed in a two-step process. In thefirst step, a portion of the bulk layer is deposited at an off-normalangle of incidence to achieve a desired c-axis tilt. Once the c-axistilt is established, the remainder of the bulk layer is deposited atnormal incidence. Despite being deposited at normal or approximatelynormal incidence, the remaining bulk layer tends to adopt the c-axistilt of the previously deposited crystal layer. The bulk layer may bedeposited directly on a substrate or may be deposited on a substratewith a seed layer.

Depositing a crystalline bulk layer at about normal incidence andachieving a desired c-axis tilt reduces waste and processing efficiency.For example, deposition at normal incidence may be done without acollimator, reducing material losses of the bulk layer material due todeposition on the collimator. Accordingly, more bulk layer material maybe transferred directly onto the substrate. Further, the frequency ofcleaning off built-up deposits on the collimator, or collimatorreplacement, may be reduced. In addition, deposition at normal incidencemay be faster, and may be performed using standard equipment and processconditions. These and other advantages will be readily understood bythose of skill in the art.

The methods of the present disclosure provide for preparing a structurehaving a crystalline bulk layer c-axis tilt of a preselected angle. Thedesired c-axis tilt depends on the intended purpose, use, and effect ofthe bulk layer. Changing the c-axis angle of inclination for hexagonalcrystal structure piezoelectric materials causes variation in shear andlongitudinal coupling coefficients, as shown in FIG. 1. FIG. 1 embodiesplots of shear coupling coefficient (K_(s)) and longitudinal couplingcoefficient (K_(l)) each as a function of c-axis angle of inclinationfor AlN. It can be seen that a maximum electromechanical couplingcoefficient of shear mode resonance in AlN is obtained at a c-axis angleof inclination of about 35 degrees, that a pure shear response (withzero longitudinal coupling) is obtained at a c-axis angle of inclinationof about 46 degrees, and that the shear coupling coefficient exceeds thelongitudinal coupling coefficient for c-axis angle of inclination valuesin a range from about 19 degrees to about 63 degrees. The longitudinalcoupling coefficient is also zero at a c-axis angle of inclination of 90degrees, but it is impractical to grow AlN at very steep c-axisinclination angles. Similar behavior is expected for other piezoelectricmaterials, although the specific angle positions may vary. Forelectroacoustic resonators intended to operate in liquids or otherviscous media, it would be desirable to provide piezoelectric films witha c-axis tilt angle sufficient to provide a shear coupling coefficientthat exceeds a longitudinal coupling coefficient—in certain embodiments,at a c-axis tilt angle in which the longitudinal coupling coefficientapproaches zero, or at a c-axis-tilt angle at or near a value whereshear coupling is maximized. Thus, for an electroacoustic resonatorincluding an AlN piezoelectric layer, it would be desirable to provide ac-axis tilt angle in a range of from about 19 degrees to about 63degrees, and particularly desirable to provide a c-axis tilt anglebetween 35 and 46 degrees. Other c-axis tilt angles may be desirable forother purposes or when materials other than AlN are used for deposition.

The shear coupling coefficient for a bulk acoustic wave resonatorcomprising a bulk layer of AlN exceeds the longitudinal couplingcoefficient for c-axis angle of inclination values in a range of fromabout 19 degrees to about 63 degrees. A greater difference between theshear mode and longitudinal coupling is achieved approximately between30 degrees and 52 degrees, and a pure shear mode resonance response(with zero longitudinal coupling) can be obtained at a c-axis angle ofinclination of about 46 degrees. Therefore, it would be desirable to beable to prepare an AlN bulk layer with a c-axis tilt of between about 30degrees and about 52 degrees, between about 32 degrees and about 50degrees, between about 35 degrees and about 48 degrees, or about 46degrees. In some embodiments, shear mode excitation may be increased bydepositing a bulk layer with a c-axis tilt of about 30 degrees to about52 degrees, about 32 degrees to about 50 degrees, or about 35 degrees toabout 48 degrees. Other angles of the c-axis tilt may also be useful inother embodiments. For example, c-axis tilts of about 30 degrees toabout 45 degrees, about 32 degrees, or about 90 degrees could be ofinterest in some embodiments.

The term “c-axis” is used here to refer to the (002) direction of adeposited crystal with a hexagonal wurtzite structure. The c-axis istypically the longitudinal axis of the crystal.

The terms “c-axis tilt,” “c-axis orientation,” and “c-axis incline” areused here interchangeably to refer to the angle of the c-axis relativeto a normal of the surface plane of the deposition substrate.

When referring to c-axis tilt or c-axis orientation, it should beunderstood that even if a single angular value is given, the crystals ina deposited crystal layer (e.g., a seed layer or a bulk layer) mayexhibit a distribution of angles. The distribution of angles typicallyapproximately follows a normal (e.g., Gaussian) distribution that can begraphically demonstrated, for example, as a two-dimensional plotresembling a bell-curve, or by a pole figure.

The term “incidence angle” is used here to refer to the angle at whichatoms are deposited onto a substrate, measured as the angle between thedeposition pathway and a normal of the surface plane of the substrate.

The term “substrate” is used here to refer to a material onto which aseed layer or a bulk layer may be deposited. The substrate may be, forexample, a wafer, or may be a part of a resonator device complex orwafer, which may also include other components, such as an electrodestructure arranged over at least a portion of the substrate. A seedlayer is not considered to be “a substrate” in the embodiments of thisdisclosure.

When referring to deposition of crystals “on a substrate,” there may beintervening layers (e.g., a seed layer) between the substrate and thecrystals. However, the expressions “directly on a substrate” or “on thesurface of the substrate” are intended to exclude any interveninglayers.

The term “seed layer” is used here to refer to a crystalline layer thatis dominated by (103) texture with little or no in-plane alignment alongthe (002) direction, and onto which a bulk material layer may bedeposited.

The term “bulk layer” is used here to refer to a crystalline layer thatexhibits primarily (002) texture. The bulk layer may be formed in one ormore steps. Reference to the bulk layer in this disclosure refers to theentire bulk layer, whether the bulk layer is formed in a single step,two steps, or more than two steps. The term “first portion” is used inthis disclosure to refer to a first deposited part (e.g., layer) of thebulk layer that exhibits primarily (002) texture.

The term “substantially” as used here has the same meaning as “nearlycompletely,” and can be understood to modify the term that follows by atleast about 90%, at least about 95%, or at least about 98%.

The terms “parallel” and “substantially parallel” with regard to thecrystals refer to the directionality of the crystals. Crystals that aresubstantially parallel not only have the same or similar c-axis tilt butalso point in the same or similar direction.

The term “about” is used here in conjunction with numeric values toinclude normal variations in measurements as expected by persons skilledin the art, and is understood have the same meaning as “approximately”and to cover a typical margin of error, such as ±5% of the stated value.

All scientific and technical terms used herein have meanings commonlyused in the art unless otherwise specified. The definitions providedherein are to facilitate understanding of certain terms used frequentlyherein and are not meant to limit the scope of the present disclosure.

As used here, the singular forms “a”, “an”, and “the” encompassembodiments having plural referents, unless the content clearly dictatesotherwise.

As used here, the term “or” is generally employed in its sense including“and/or” unless the content clearly dictates otherwise. The term“and/or” means one or all of the listed elements or a combination of anytwo or more of the listed elements.

As used here, “have”, “having”, “include”, “including”, “comprise”,“comprising” or the like are used in their open-ended sense, andgenerally mean “including, but not limited to.” It will be understoodthat “consisting essentially of,” “consisting of,” and the like aresubsumed in “comprising” and the like. As used herein, “consistingessentially of,” as it relates to a composition, product, method or thelike, means that the components of the composition, product, method orthe like are limited to the enumerated components and any othercomponents that do not materially affect the basic and novelcharacteristic(s) of the composition, product, method or the like.

The words “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from the scopeof the disclosure, including the claims.

The recitations of numerical ranges by endpoints include all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, 5, etc. or 10 or less includes 10, 9.4, 7.6, 5, 4.3, 2.9, 1.62,0.3, etc.). Where a range of values is “up to” a particular value, thatvalue is included within the range.

Any direction referred to here, such as “top,” “bottom,” “left,”“right,” “upper,” “lower,” and other directions and orientations aredescribed herein for clarity in reference to the figures and are not tobe limiting of an actual device or system or use of the device orsystem. Devices or systems as described herein may be used in a numberof directions and orientations.

The present disclosure relates in various aspects to bulk acoustic waveresonator structures and to methods for fabricating such resonatorstructures. As compared to conventional resonator structures,fabrication methods, and deposition systems, various embodiments of thepresent disclosure include or enable inclined c-axis piezoelectric filmswith a preselected c-axis tilt angle with an increased range ofselectable angles. The inclined c-axis piezoelectric films may alsoexhibit improved mechanical quality factor, reduced acoustic losses,and/or reduced ohmic (electrical) losses. The inclined c-axispiezoelectric films may be fabricated over large areas (e.g., large areasubstrates) with increased uniformity of c-axis tilt angle. The methodsfor making the inclined c-axis piezoelectric films may be more efficientor reduce waste relative to prior art methods for preparing inclinedc-axis piezoelectric films.

The methods described herein include a bulk layer deposition processwith two or more steps. The process may further include depositing aseed layer on a substrate or may include deposition of the bulk layerdirectly on the substrate (without an intervening seed layer).

Referring now to FIGS. 2A-2D and 3A-3D, schematic diagrams for two-stepbulk layer deposition processes are shown. FIGS. 2A-2D illustrate aprocess in which the bulk layer 40 is directly deposited on a substrate4 without a seed layer. The first growth step (shown in FIG. 2A)includes ejection of metal atoms from a target 2 of a linear sputteringapparatus to react with a gas species forming a deposition flux 10 to bereceived by the substrate 4. The deposition system may include amulti-aperture collimator 17 arranged between the target and thesubstrate. The deposition flux 10 may be directed through the apertures18 of the collimator 17 to help control the incidence angle duringdeposition. The deposition flux 10 arrives at the substrate 4 at a firstincidence angle α, forming a first portion 41 of the bulk layer 40(shown in FIG. 2B). The crystals of the first portion 41 of the bulklayer 40 have a c-axis tilt 41 y.

In a second growth step (shown in FIG. 2C), metal atoms are ejected fromtarget 2 to react with a gas species and to be received by the firstportion 41 already deposited on the substrate 4. In the second growthstep, the target 2 may be positioned such that the second incidenceangle is smaller than the first incidence angle α (e.g., is betweennormal and the first incidence angle α). For example, the secondincidence angle β may be about 0 degrees (i.e., normal to the surface ofthe substrate 4). The deposition flux 10 in the second growth step forma second portion 42 of the bulk layer 40 (shown in FIG. 2D). Thecrystals of the second portion 42 of the bulk layer 40 have a c-axistilt 42 y. The second growth step may be done without a collimator.

According to an embodiment, the c-axis tilt 42 y of the second portion42 follows or substantially follows the c-axis tilt 41 y of the firstportion 41 of the bulk layer 40. In some embodiments, the c-axis tilt 41y, 42 y of the first and second portions 41, 42 aligns or at leastsubstantially aligns with the first incidence angle α used during thefirst growth step. The resulting bulk layer crystals of the firstportion 41 and second portion 42 may be substantially parallel to oneanother and at least substantially align with the desired c-axis tilt.The resulting bulk layer crystals of the first portion 41 and secondportion 42 may also be substantially parallel within each portion. Forexample, at least 50%, at least 75%, or at least 90% of the crystals ofthe first portion 41 may have a c-axis tilt 41 y that is within 0degrees to 10 degrees of the average c-axis tilt, and a direction thatis within 0 degrees to 45 degrees, or within 0 degrees to 20 degrees ofthe average crystal direction. Similarly, at least 50%, at least 75%, orat least 90% of the crystals of the second portion 42 may have a c-axistilt 42 y that is within 0 degrees to 10 degrees of the average c-axistilt, and a direction that is within 0 degrees to 45 degrees, or within0 degrees to 20 degrees of the average crystal direction.

FIGS. 3A-3D illustrate a process in which the bulk layer 50 is depositedon a seed layer 31, which has been deposited on the substrate 4. A firstportion 51 of a crystalline bulk layer 50 (FIG. 3B) is deposited ontothe seed layer 31 in the first growth step shown in FIG. 3A. In thefirst growth step, the atoms in the deposition flux 10 (metal atomsreacted with a gas) are deposited at a first incidence angle α. In asecond growth step, shown in FIG. 3C, the atoms in the deposition flux10 are deposited at a second incidence angle β, resulting in the secondportion 52 of the bulk layer 50. The second incidence angle β may besmaller than the first incidence angle α. For example, the secondincidence angle β may be about 0 degrees (i.e., normal to the surface ofthe substrate 4). The crystals of the second portion 52 of the bulklayer 50 have a c-axis tilt 52 y.

According to an embodiment, the c-axis tilt 52 y of the second portion52 follows or substantially follows the c-axis tilt 51 y of the firstportion 51 of the bulk layer 50. In some embodiments, the c-axis tilt 51y, 52 y of the first and second portions 51, 52 aligns or at leastsubstantially aligns with the first incidence angle α used during thefirst growth step. The resulting bulk layer crystals of the firstportion 51 and second portion 52 may be substantially parallel to oneanother and at least substantially align with the desired c-axis tilt.The resulting bulk layer crystals of the first portion 51 and secondportion 52 may also be substantially parallel within each portion. Forexample, at least 50%, at least 75%, or at least 90% of the crystals ofthe first portion 51 may have a c-axis tilt 51 y that is within 0degrees to 10 degrees of the average c-axis tilt, and a direction thatis within 0 degrees to 45 degrees, or within 0 degrees to 20 degrees ofthe average crystal direction. Similarly, at least 50%, at least 75%, orat least 90% of the crystals of the second portion 52 may have a c-axistilt 52 y that is within 0 degrees to 10 degrees of the average c-axistilt, and a direction that is within 0 degrees to 45 degrees, or within0 degrees to 20 degrees of the average crystal direction.

Both of the methods shown in FIGS. 2A-2D and 3A-3D may employ acollimator 17 in the first growth step (see FIGS. 2A and 3A), whichincludes positioning a target 2 at an off-normal incidence relative tothe substrate 4. The second step (see FIGS. 2C and 3C), in which thesecond portion 42, 52 of the bulk layer 40, 50 is deposited at aboutnormal incidence preferably does not employ a collimator.

According to at least some embodiments of the present disclosure, thebulk layer 40, 50 has a c-axis tilt that can be pre-selected. Themethods of the present disclosure result in a bulk layer 40, 50 wherethe crystals of the bulk layer align with or substantially align withthe pre-selected c-axis tilt. In some embodiments, the distribution ofthe c-axis tilt of the crystals is such that at least 75%, at least 80%,at least 85%, at least 90%, or at least 95% of the crystals in the bulklayer have a c-axis tilt that is within a range of the pre-selectedc-axis tilt, such as within 1 to 10 degrees, within 1 to 8 degrees,within 1 to 5 degrees, or within 1 to 3 degrees of the pre-selectedc-axis tilt.

Without wishing to be bound by theory, it is hypothesized that thedeposition conditions during at least one growth step may be selectedsuch that the deposition conditions retard the surface mobility of theatoms being deposited. The deposition conditions that may have a surfacemobility retarding effect include many variables. These variables may beselected so that surface mobility is decreased to the point that thec-axis tilt of the seed layer and/or bulk layer can be controlled. Thesurface mobility of the atoms is a result of the variables as a whole,and not necessarily any single variable alone. When compared toconventional methods and deposition conditions for depositing bulklayers, each of the variables may be somewhat different, or only some ofthe variables may be different while others may remain the same as inconventional methods. Because surface mobility of atoms is difficult todetermine directly, the combination of appropriate conditions may bedetermined based on the ability to change the c-axis tilt of theresulting crystalline layer beyond the angle(s) that is typicallyavailable for a deposited material due to crystallographic restrictions.For example, in the case of AlN, the ability to produce a crystallinelayer with a c-axis tilt aligned along 32 degrees (with distribution ofangles ranging from about 25 to about 35 degrees) or greater mayindicate deposition conditions that favor kinetics over thermodynamicsand that allow crystal growth to respond to changes in the depositionenvironment. Also, the ability to deposit a bulk layer directly onto asubstrate (without a seed layer) at a c-axis tilt angle aligned along 20degrees, above 25 degrees, above 30 degrees, or above 35 degrees mayindicate deposition conditions that favor kinetics over thermodynamics.The crystals in the bulk layer may be aligned or substantially alignedover the area of the substrate (e.g., over the entire deposition area).

According to an embodiment, an initial (e.g., first) portion of the bulklayer may be deposited under the initial deposition conditions at anoff-normal incidence angle such that crystals in the resulting initialbulk layer have the desired c-axis tilt. The remaining bulk layer (e.g.,a second or subsequent portion) may be deposited under the same initialdeposition conditions, or under different deposition conditions (e.g.,conditions typically used for bulk layer deposition).

In some embodiments, bulk acoustic wave resonator structures and methodsfor fabricating such resonator structures include deposition of the bulklayer directly on the substrate (without a seed layer). Improvedcoupling efficiency and mechanical quality factor may result from theelimination of the seed layer, even when the seed layer might be formedfrom the same material as the bulk layer. In some embodiments, theresonator structures are formed by depositing the bulk layer directly onthe substrate, and exhibit an improved mechanical quality factor,reduced acoustic losses, and/or reduced ohmic (electrical) losses. Theresonator structures also exhibit increased uniformity of the c-axistilt angle over large areas.

According to at least some embodiments of the present disclosure, thec-axis tilt of the bulk layer may be adjusted by depositing a firstportion of the bulk layer at the desired angle under initial depositionconditions. According to some embodiments, the initial depositionconditions are such that they retard the surface mobility of atoms whilethe bulk layer is being deposited. In at least some embodiments, thebulk layer with a pre-selected c-axis tilt can be deposited under theinitial deposition conditions without first depositing a seed layer.

In some embodiments, the initial deposition conditions during the firstgrowth step may include one or more of incidence angle, pressure,temperature, distance from the target to the substrate, and gas ratio.The incidence angle may be an off-normal incidence. For example, theincidence angle may be greater than 10 degrees, greater than 27 degrees,greater than 30 degrees, greater than 32 degrees, greater than 33degrees, greater than 34 degrees, greater than 35 degrees, greater than36 degrees, or greater than 40 degrees. The incidence angle may be up toabout 85 degrees, up to about 75 degrees, up to about 65 degrees, up toabout 56 degrees, up to about 52 degrees, up to about 50 degrees, up toabout 49 degrees, or up to about 48 degrees. Illustrative incidenceangles include 35 degrees, 40 degrees, 43 degrees, and 46 degrees. Insome embodiments, the incidence angle is less than 32 degrees or greaterthan 40 degrees.

The pressure during the first growth step may be at least about 0.5mTorr, at least about 1 mTorr, or at least about 1.5 mTorr. The pressuremay be up to about 10 mTorr, up to about 8 mTorr, up to about 6 mTorr,up to about 5 mTorr, or up to about 4 mTorr. In some embodiments, thepressure is below 5 mTorr. For example, the pressure may be about 2mTorr, about 2.5 mTorr, about 3 mTorr, about 3.5 mTorr, or about 4mTorr. The temperature may be at least about 20° C., at least about 50°C., or at least about 100° C. The temperature may be up to about 300°C., up to about 250° C., or up to about 200° C. In some embodiments, thedeposition process may generate heat but the deposition chamber in whichthe deposition is performed is not heated by a heater (i.e., is notintentionally heated).

The distance from the target to the substrate during the first growthstep may be at least about 50 mm, at least about 75 mm, at least about80 mm, or at least about 90 mm. The distance may be up to about 200 mm,up to about 150 mm, up to about 130 mm, or up to about 120 mm. In someembodiments, the distance from the target to the substrate duringdeposition may be about 108 mm to about 115 mm.

The gases in the vapor space of the deposition system may be selectedbased on the intended composition of the deposited layer, and mayinclude argon and a gas that reacts with the deposited atoms, such asnitrogen or oxygen. The gas ratio of argon to reacting gas (e.g.,nitrogen) in the vapor space may be from about 1:10 to about 10:10, fromabout 2:10 to about 8:10, or about 4:10.

In some embodiments, bulk acoustic wave resonator structures and methodsfor fabricating such resonator structures include deposition of the bulklayer onto a seed layer. A seed layer can be used to provide a texturedsurface for depositing the bulk layer. The seed layer may exhibitdifferent textures, most notably (103) and (002). A directionaldeposition flux and competitive column growth may result in a bulk layerwith a c-axis substantially oriented along the deposition flux.

In certain embodiments, the crystalline seed layer is compositionallymatched to the hexagonal crystal structure piezoelectric material bulklayer. In some embodiments, the thickness of the crystalline seed layeris no greater than about 20%, no greater than about 15%, or no greaterthan about 10% of the combined thickness of the bulk layer and seedlayer. In certain embodiments, the seed layer includes a thickness in arange of from about 500 Angstroms to about 2,000 Angstroms, and (for ahexagonal crystal structure seed material such as AlN) may include adominant (103) texture.

The seed layer may be prepared (e.g., deposited onto a substrate)according to known methods and conditions, such as those discussed inU.S. patent application Ser. No. 15/293,071 entitled “Methods forFabricating Acoustic Structure with Inclined C-Axis Piezoelectric Bulkand Crystalline Seed Layers.” In some embodiments, the seed layer isdeposited at a pressure of 8 mTorr or greater, 10 mTorr or greater, or12 mTorr or greater, and up to 25 mTorr, up to 20 mTorr, or up to 18mTorr.

According to some embodiments, the bulk layer is deposited onto the seedlayer in a first growth step during initial deposition conditions, whichmay include one or more of incidence angle, pressure, temperature,distance from the target to the substrate, and gas ratio.

The incidence angle of first growth step may be an off-normal incidence.For example, the incidence angle may be greater than 10 degrees, greaterthan 27 degrees, greater than 30 degrees, greater than 32 degrees,greater than 33 degrees, greater than 34 degrees, greater than 35degrees, greater than 36 degrees, or greater than 40 degrees. Theincidence angle may be up to about 85 degrees, up to about 75 degrees,up to about 65 degrees, up to about 56 degrees, up to about 52 degrees,up to about 50 degrees, up to about 49 degrees, or up to about 48degrees. Illustrative incidence angles include 35 degrees, 40 degrees,43 degrees, and 46 degrees. In some embodiments, the incidence angle isless than 32 degrees or greater than 40 degrees.

The pressure during the first growth step may be at least about 0.5mTorr, at least about 1 mTorr, or at least about 1.5 mTorr. The pressuremay be up to about 10 mTorr, up to about 8 mTorr, or up to about 6mTorr. In some embodiments, the pressure is below 5 mTorr. For example,the pressure may be about 2 mTorr, about 2.5 mTorr, about 3 mTorr, about3.5 mTorr, or about 4 mTorr. The temperature may be at least about 20°C., at least about 50° C., or at least about 100° C. The temperature maybe up to about 300° C., up to about 250° C., or up to about 200° C. Insome embodiments, the deposition process may generate heat but thedeposition chamber in which the deposition is performed is not heated bya heater (i.e., is not intentionally heated).

The distance from the target to the substrate during deposition may beat least about 50 mm, at least about 75 mm, at least about 80 mm, or atleast about 90 mm. The distance may be up to about 200 mm, up to about150 mm, up to about 130 mm, or up to about 120 mm. In some embodiments,the distance from the target to the substrate during deposition may beabout 108 mm to about 115 mm.

The gases in the vapor space of the deposition system may be selectedbased on the intended composition of the deposited layer, and mayinclude argon and a gas that reacts with the deposited atoms, such asnitrogen or oxygen. The gas ratio of argon to reacting gas (e.g.,nitrogen) in the vapor space may be from about 1:10 to about 10:10, fromabout 2:10 to about 8:10, or about 4:10.

The surface onto which the seed layer, the bulk layer, or a portion ofthe bulk layer is deposited may optionally be roughened prior todeposition. For example, the surface of the substrate, the surface ofthe seed layer, or the surface of the first portion of the bulk layermay be roughened. The surface may be roughened by, for example, atomicbombardment. Roughening of the surface may improve the ability of thesubsequently grown bulk layer crystals to orient during the deposition.Without wishing to be bound by theory, it is believed that rougheningthe surface causes shadowing effects, which may help favor orientationof the crystals toward the angle of deposition. The surface of thesubstrate may be roughened by, for example, atomic bombardment, creating“hills” and “valleys” on the surface.

According to certain embodiments, a second portion of the bulk layer isdeposited at normal incidence, or at an incidence angle that is betweenthe first incidence and a normal to the plane of the substrate. Theability to deposit a portion, or a majority, of the bulk layer (e.g.,the second portion of the bulk layer) at a normal incidence may makemanufacturing faster and more efficient. In addition, depositing at anormal incidence may cause less material to be deposited on thecollimator.

The deposition conditions during the second growth step may include theincidence angle and one or more of pressure, temperature, distance fromthe target to the substrate, and gas ratio. The second incidence anglemay be about 0 degrees, up to about 5 degrees, up to about 10 degrees,up to about 15 degrees, up to about 20 degrees, up to about 25 degrees,up to about 30 degrees, up to about 35 degrees, or up to about 40degrees.

The deposition conditions during the second growth step may be the sameas the initial deposition conditions or may be different. The depositionconditions during the second growth step may include, for example, apressure of about 0.5 mTorr to about 15 mTorr, from about 0.8 mTorr toabout 10 mTorr, or from about 1 mTorr to about 5 mTorr. In someembodiments, the pressure is below 5 mTorr. For example, the pressuremay be about 2 mTorr, about 2.5 mTorr, about 3 mTorr, about 3.5 mTorr,or about 4 mTorr. The temperature during the second growth step mayrange from about 20° C. to about 300° C., from about 50° C. to about250° C., or from about 100° C. to about 200° C. The gas ratio of argonto reacting gas (e.g., nitrogen) in the vapor space may be from about1:10 to about 10:10, from about 2:10 to about 8:10, or about 4:10.

The materials used in the first and second growth steps may be the sameor may be different.

Suitable materials for the bulk layer include piezoelectric materials orother metallic materials with a high melting point. In some embodimentsthe material includes a metal nitride, such as aluminum nitride,titanium nitride, hafnium nitride, tantalum nitride, zirconium nitride,vanadium nitride, niobium nitride, etc. In some embodiments the materialincludes a metal oxide, such as zinc oxide, tungsten oxide, hafniumoxide, molybdenum oxide, etc. In some embodiments, the materialcomprises a metal oxynitride, such as hafnium oxynitride, titaniumoxynitride, tantalum oxynitride, etc. In some embodiments the materialincludes a metal carbide such as titanium carbide, niobium carbide,tungsten carbide, tantalum carbide, etc. In some embodiments thematerial is a refractory metal, such as zirconium, hafnium, tungsten,molybdenum, etc. The bulk layer may comprise a combination of two ormore of the material described above.

In certain embodiments, a hexagonal crystal structure piezoelectricmaterial bulk layer comprises a c-axis having an orientationdistribution predominantly in a range of from 12 degrees to 52 degrees,or in a range of from 27 degrees to 37 degrees, or in a range of from 75degrees to 90 degrees, relative to normal of a face of a substrate orwafer supporting the hexagonal crystal structure piezoelectric materialbulk layer.

The distribution of the c-axis orientation of the hexagonal crystalstructure piezoelectric material bulk layer may be normal or bimodal. Ina preferred embodiment, the distribution is normal. In certainembodiments, less than about 30%, less than about 25%, or less thanabout 20% of the c-axis orientation distribution of the hexagonalcrystal structure piezoelectric material bulk layer is in a range offrom 0 degrees to 25 degrees relative to normal of a face of thesubstrate. In certain embodiments, less than about 30%, less than about25%, or less than about 20% of the c-axis orientation distribution ofthe hexagonal crystal structure piezoelectric material bulk layer is ina range of from 45 degrees to 90 degrees relative to normal of a face ofthe substrate. At least 60%, at least 65%, at least 70%, at least 75%,at least 80%, at least 85%, or at least 90% of the c-axis orientationdistribution of the hexagonal crystal structure piezoelectric materialbulk layer may be in a range of 25 degrees to 45 degrees. In someembodiments, at least 60%, at least 65%, at least 70%, at least 75%, atleast 80%, at least 85%, or at least 90% of the c-axis orientationdistribution of the hexagonal crystal structure piezoelectric materialbulk layer is in a range of 30 degrees to 40 degrees.

In certain embodiments, the hexagonal crystal structure piezoelectricmaterial bulk layer (combined first and second portions) may have athickness of about 1,000 Å (Angstrom) or greater, about 2,000 Å orgreater, about 3,000 Å or greater, about 4,000 Å or greater, about 6,000Å or greater, or about 10,000 Å or greater. The thickness of thehexagonal crystal structure piezoelectric material bulk layer may be upto about 30,000 Å, up to about 26,000 Å, or up to about 20,000 Å. Suchhexagonal crystal structure piezoelectric material bulk layer preferablyincludes substantially uniform thickness, nanostructure, andcrystallinity properties, with controlled stress and densely packedcolumnar grains or recrystallized grain structure. In certainembodiments, a crystalline seed layer includes a thickness in a range offrom 500 Å to 2,000 Å, and (for a hexagonal crystal structurepiezoelectric material such as AlN) may include a dominant (103)texture. In other embodiments, the structure does not include a seedlayer.

According to some embodiments, an acoustic resonator structure isprepared in a deposition system where at least one wafer comprising asubstrate is received by a support surface. An acoustic reflectorstructure may be arranged over the substrate, and an electrode structurearranged over at least a portion of the acoustic reflectorstructure—such as may be useful to produce at least one solidly mountedbulk acoustic wave resonator device. In certain other embodiments, theat least one wafer includes a substrate defining a recess, a supportlayer is arranged over the recess, and an electrode structure isarranged over the support layer—such as may be useful to produce atleast one film bulk acoustic wave resonator device. In describing themethod here, deposition simply onto “a substrate” may be described.However, it is to be understood that the substrate may be a part of aresonator device complex or wafer, which may also include othercomponents, such as an electrode structure arranged over at least aportion of the substrate.

In one aspect, the disclosure relates to a method for fabricating atleast one acoustic resonator structure, wherein a growth step (e.g., afirst growth step and a second growth step) includes deposition of ahexagonal crystal structure bulk layer on a substrate. The depositedmaterial may be piezoelectric material. The growth step includesejection of metal atoms from a target surface of a linear sputteringapparatus to react with a gas species and to be received by thesubstrate.

The acoustic resonator structures prepared according to methods of thepresent disclosure, and devices made using such acoustic resonatorstructures, may be more robust and exhibit improved mechanical stabilityas compared to structures prepared using previously known methods.

The seed and bulk layers of the present disclosure can be prepared inany suitable deposition system. One example of a suitable depositionsystem is described in U.S. patent application Ser. No. 15/293,063entitled “Deposition System for Growth of Inclined C-Axis PiezoelectricMaterial Structures.” The main aspects of the deposition system aresummarized below. However, the methods of the present disclosure are notparticularly limited by the system used, and other suitable systems mayalso be used.

The crystalline layers of the present disclosure can be prepared in adeposition system incorporating a multi-aperture collimator arrangedbetween a target surface of a linear sputtering apparatus and asubstrate table that supports one or more wafers or substrates forreceiving sputter-deposited material. In some embodiments, the wafers orsubstrates may be placed on a stationary pedestal for deposition atnormal incidence (0 degrees).

An exemplary deposition system is shown in FIG. 4, which is an upperexterior perspective view of the reactor 100 of the deposition systemfor growing hexagonal crystal structure piezoelectric materials. Thereactor 100 includes first, second, and third tubular portions 102, 120,108 for housing various elements used for depositing material onto asubstrate. FIG. 5 depicts an upper perspective view of some of theelements of the reactor 100, including a linear sputtering apparatus154, a translation track 115 for translating a movable substrate tablefor supporting multiple substrates, and a collimator assembly 170.

The target surface may be non-parallel to the substrate table, and theintermediately arranged collimator may be non-parallel to both thetarget surface and the substrate table. The collimator and the substratetable are preferably both capable of movement (e.g., translation) duringsputtering, and at least one of the substrate table or the collimator ispreferably biased to an electrical potential other than ground. Thesystem may be used to grow (e.g., deposit) a crystalline seed layer,followed by growth of a hexagonal crystal structure piezoelectricmaterial bulk layer over the crystalline seed layer under conditionsthat differ from the seed layer deposition. Alternatively, according toa method of the present disclosure, the bulk layer may be depositeddirectly onto the substrate without first depositing a seed layer.

According to an embodiment, the bulk layer is grown (e.g., deposited)using a single sputtering apparatus. The growth step (e.g., the firstand/or second growth step) may be performed with a deposition systemutilizing a linear sputtering apparatus, a substrate table that istranslatable between different positions within the linear sputteringapparatus, and a collimator arranged between the substrate table and thelinear sputtering apparatus. A hexagonal crystal structure piezoelectricmaterial bulk layer may be grown in an enclosure in which subatmosphericconditions may be generated using at least one vacuum pumping element,and a wafer or substrate supporting the bulk layer may be translatedwithin the enclosure.

In some embodiments, the bulk layer is grown (e.g., deposited) in two ormore steps which may be performed with a deposition system utilizingmultiple linear sputtering apparatuses, a substrate table that istranslatable between different positions proximate to different linearsputtering apparatuses. A collimator may be arranged between thesubstrate table and the respective linear sputtering apparatuses. Forexample, a crystalline seed layer and/or a first portion of a hexagonalcrystal structure piezoelectric material bulk layer may be grown byreactive sputtering at a first station using a first collimatoraccording to a first growth step, and a hexagonal crystal structurepiezoelectric material bulk layer (or a second portion of the bulklayer) may be grown by reactive sputtering at a second station without acollimator in a second growth step. Both stations may be located in asingle enclosure in which subatmospheric conditions may be generatedusing at least one vacuum pumping element, and a wafer or substratesupporting the respective layers may be moved between the stationswithout need for removal from subatmospheric conditions. In someembodiments, different process conditions and/or different angularpositions between target surfaces, collimator, and a wafer or supportsurface may be used in the first and second growth steps.

The deposition system suitable for growing tilted c-axis hexagonalcrystal structure piezoelectric material may include a linear sputteringapparatus, a multi-aperture collimator, and a translatable substratetable having a support surface arranged non-parallel to a target surfaceof the sputtering apparatus, with the substrate table and/or thecollimator being electrically biased to a potential other than ground.The linear sputtering apparatus, which may include a linear magnetron ora linear ion beam sputtering apparatus, includes a target surfaceconfigured to eject metal (e.g., aluminum or zinc) atoms, with thetarget surface being non-parallel to (e.g., oriented at 0 to less than90 degrees from) the support surface. The collimator may also bearranged non-parallel to the support surface. In certain embodiments,e.g., during the first growth step, a target surface is arranged at afirst nonzero angle relative to a support surface, and a collimator isarranged at a second nonzero angle relative to the support surface,wherein the first nonzero angle is greater than the second nonzeroangle. Metal atoms ejected from the target surface react with a gasspecies contained in a gas-containing environment to form the materialto be deposited (e.g., piezoelectric material). For example, aluminumatoms ejected from an aluminum or aluminum-containing target surface mayreact with nitrogen gas species to form aluminum nitride, or zinc atomsejected from a zinc or zinc-containing target surface may react withoxygen gas species to form zinc oxide.

The support surface of the substrate table may be configured to receiveone or more wafers to be used as deposition substrates, preferablyhaving a diameter in a range of at least about 50 mm, about 100 mm, orabout 150 mm. The substrate table may be coupled to a movable element(e.g., a translation element) configured to move the substrate tableduring operation of the linear sputtering apparatus. Movement of thesubstrate table may promote uniform material deposition over large areasby preventing localized material deposition regions of differentthicknesses. The exemplary collimator includes multiple guide membersarranged non-parallel to the support surface, such as a plurality oflongitudinal members and a plurality of transverse members, that form agrid defining multiple collimator apertures. Electrical biasing of thesubstrate table and/or the collimator to a potential other than groundenhances control of material deposition during operating of thesputtering apparatus. Collimator biasing may also influencemicrostructure development of tilted c-axis piezoelectric bulk materialin a bulk acoustic wave resonator device. The substrate table and thecollimator may be independently biased to electrical potentials otherthan ground. Separate guide members of the collimator may also beelectrically biased differently relative to one another. The collimatormay be configured to translate during operation of the linear sputteringapparatus to prevent formation of a “shadow” pattern that couldotherwise be formed on a surface receiving deposited piezoelectricmaterial. A deposition aperture may be arranged between the collimatorand the substrate table.

According to at least some embodiments, the c-axis tilt of the resultingbulk layer is the same as the preselected angle or is within a range ofthe incidence angle and/or the preselected angle. For example, thec-axis tilt of the resulting bulk layer may be within 1 degree, within 2degrees, within 3 degrees, within 5 degrees, within 10 degrees, orwithin 15 degrees of the incidence angle and/or the pre-selected c-axistilt. The distribution of the c-axis tilt of the bulk layer crystals maybe such that at least 75%, at least 80%, at least 85%, at least 90%, orat least 95% of the crystals in the bulk layer have a c-axis tilt thatis within a range, such as within 1 degree, within 2 degrees, within 3degrees, within 5 degrees, within 10 degrees, or within 15 degrees, ofthe incidence angle and/or the pre-selected c-axis tilt.

In certain embodiments, a substrate table and/or collimator isconfigured to translate during the first and/or second growth steps topromote uniform material deposition. An electrode structure may beformed over at least one portion of the hexagonal crystal structurepiezoelectric material bulk layer to form at least one bulk acousticwave resonator device. An active region of bulk acoustic wave resonatordevice is provided in an area in which the hexagonal crystal structurepiezoelectric material bulk layer is arranged between a first electrodestructure and a second electrode structure. Such growth steps may beperformed using a single sputtering apparatus or a deposition systemutilizing multiple linear sputtering apparatuses, a substrate table thatis translatable between different positions proximate to differentlinear sputtering apparatuses, and optionally a collimator arrangedbetween the substrate table and the respective linear sputteringapparatuses. In certain embodiments, at least one resonator devicecomplex, over which the hexagonal crystal structure piezoelectricmaterial bulk layer is deposited, is diced into a plurality of chips,such as solidly mounted bulk acoustic wave resonator chips or film bulkacoustic wave resonator chips.

In another aspect of the present disclosure, a method for fabricating atleast one resonator structure includes use of a first station containinga first linear sputtering apparatus including a first target surface,and use of a second station containing a second linear sputteringapparatus including a second target surface. At least one waferstructure supported by a substrate table is moved to the first stationin which a first subatmospheric pressure condition is generated, a firstgrowth step is performed to deposit a first portion of a hexagonalcrystal structure piezoelectric material bulk layer over the at leastone wafer structure, the at least one wafer structure supported by thesubstrate table is moved to the second station in which a secondsubatmospheric pressure is generated, and a second growth step isperformed to deposit a second portion of a hexagonal crystal structurepiezoelectric material bulk layer over the first portion of the bulklayer, wherein the second portion of the hexagonal crystal structurepiezoelectric material bulk layer has a c-axis orientation distributionthat is substantially similar to the c-axis orientation distribution ofthe first portion of the hexagonal crystal structure piezoelectricmaterial bulk layer. The first growth step includes ejection of metalatoms from the first target surface to (i) transit through a firstdeposition aperture (optionally preceded by transit through a firstcollimator including multiple first collimator apertures), and (ii)react with a gas species and be received by the at least one waferstructure, to deposit the first portion of the hexagonal crystalstructure piezoelectric material bulk layer. The second growth stepincludes ejection of metal atoms from the second target surface to (i)transit through a second deposition aperture, and (ii) react with a gasspecies and be received by the first portion, to deposit the secondportion of the hexagonal crystal structure piezoelectric material bulklayer. The second growth step may or may not involve using a collimator.In certain embodiments, the first growth step is configured to yield afirst portion of the hexagonal crystal structure piezoelectric materialbulk layer having an orientation distribution predominantly within arange (e.g., within ±5 degrees or within ±10 degrees) of the preselectedangle; and the second growth step is configured to yield a secondportion of the hexagonal crystal structure piezoelectric material bulklayer including a c-axis having an orientation distributionpredominantly within a range (e.g., within ±5 degrees or within ±10degrees) of the same preselected angle. Predominantly is intended tomean at least 50%, at least about 75%, at least about 90%, or at leastabout 95% of the crystals in the layer.

In certain embodiments, the substrate table supporting the at least onewafer structure is loaded into a load lock chamber, and an initialsubatmospheric condition is generated in the load lock chamber, prior tothe moving of the at least one wafer structure supported by thesubstrate table to the first station. In certain embodiments, the firststation and the second station are arranged within a single enclosure inwhich the first subatmospheric pressure condition and the secondsubatmospheric pressure condition are generated. In other embodiments,the first station is arranged within a first chamber having anassociated first vacuum pumping element, and the second station isarranged within a second chamber having an associated second vacuumpumping element.

In certain embodiments, the substrate has a diameter of at least about50 mm (or at least about 100 mm, or at least about 150 mm) and thehexagonal crystal structure piezoelectric material bulk layer covers atleast about 50% (or at least about 75%, or at least about 90%, or atleast about 95%) of a face of the substrate. In certain embodiments,multiple bulk acoustic wave resonator devices each including an activeregion between a first electrode structure and a second electrodestructure are provided on a single substrate. Multiple bulk acousticresonator chips may be derived from such a substrate (e.g., by dicing),and may be incorporated in one or more sensors and/or fluidic devices.

In one embodiment, the deposition system is configured for growth of ahexagonal crystal structure piezoelectric material bulk layer directlyover a substrate (without first depositing a seed layer). In anotherembodiment, the deposition system is configured for growth of ahexagonal crystal structure piezoelectric material bulk layer onto aseed layer disposed on a substrate. The substrate may be a waferreceived by the support surface, wherein at least 50% (or at least 75%,or at least 90%, or at least 95%) of the hexagonal crystal structurepiezoelectric material bulk layer comprises a c-axis having anorientation distribution predominantly in a range of from 25 degrees to50 degrees (or in a subrange of from 30 degrees to 40 degrees with apeak at about 35 degrees), or greater than 10 degrees, greater than 27degrees, greater than 30 degrees, greater than 32 degrees, greater than33 degrees, greater than 34 degrees, greater than 35 degrees, greaterthan 36 degrees, or greater than 40 degrees, relative to normal of aface of a substrate or wafer received by the support surface. Theorientation distribution may be up to about 85 degrees, up to about 80degrees, up to about 75 degrees, up to about 65 degrees, up to about 56degrees, up to about 52 degrees, up to about 50 degrees, up to about 49degrees, or up to about 48 degrees. Such c-axis orientation distributionis preferably substantially uniform over the area of a large areasubstrate (e.g., having a diameter in a range of at least about 50 mm,about 100 mm, or about 150 mm), thereby enabling multiple chips havingthe same or similar acoustic wave propagation characteristics to bederived from a single substrate.

The bulk layer grown according to the methods of this disclosure have apreselected c-axis tilt angle. The selection of the preselected c-axistilt angle will depend on the desired or intended use of the resultingcrystalline bulk layer structure (e.g., an acoustic resonatorstructure). For example, the preselected angle may be any angle greaterthan 0 degrees and less than 90 degrees. It may be desirable to selectan angle that favors shear mode resonance. For example, the preselectedangle may be greater than 10 degrees, greater than 27 degrees, greaterthan 30 degrees, greater than 32 degrees, greater than 33 degrees,greater than 34 degrees, greater than 35 degrees, greater than 36degrees, or greater than 40 degrees. The preselected angle may be up toabout 85 degrees, up to about 75 degrees, up to about 65 degrees, up toabout 56 degrees, up to about 52 degrees, up to about 50 degrees, up toabout 49 degrees, or up to about 48 degrees. Exemplary preselectedangles include 35 degrees and 46 degrees. In some embodiments, thepreselected angle is less than 32 degrees or greater than 46 degrees.

The piezoelectric material films with a bulk layer according toembodiments of the present disclosure can be used in various bulkacoustic wave (“BAW”) devices, such as BAW resonators. Exemplary BAWresonators employing the piezoelectric material films of the presentdisclosure are shown in FIGS. 6-7.

FIG. 6 is a schematic cross-sectional view of a portion of a bulkacoustic wave solidly mounted resonator device 50 including apiezoelectric material bulk layer 64 embodying an inclined c-axishexagonal crystal structure piezoelectric material (e.g., AlN or ZnO) asdisclosed herein. The c-axis (or (002) direction) of the piezoelectricmaterial bulk layer 64 is tilted away from a direction normal to thesubstrate 52, as illustrated by two arrows superimposed over thepiezoelectric material bulk layer 64. The resonator device 50 includesthe substrate 52 (e.g., typically silicon or another semiconductormaterial), an acoustic reflector 54 arranged over the substrate 52, thepiezoelectric material bulk layer 64, and bottom and top side electrodes60, 68. The bottom side electrode 60 is arranged between the acousticreflector 54 and the piezoelectric material bulk layer 64, and the topside electrode 68 is arranged along a portion of an upper surface 66 ofthe piezoelectric material bulk layer 64. An area in which thepiezoelectric material bulk layer 64 is arranged between overlappingportions of the top side electrode 68 and the bottom side electrode 60is considered the active region 70 of the resonator device 50. Theacoustic reflector 54 serves to reflect acoustic waves and thereforereduce or avoid their dissipation in the substrate 52. In certainembodiments, the acoustic reflector 54 includes alternating thin layers56, 58 of materials of different acoustic impedances (e.g., SiOC, Si₃N₄,SiO₂, AlN, and Mo), optionally embodied in a Bragg mirror, depositedover the substrate 52. In certain embodiments, other types of acousticreflectors may be used. Steps for forming the resonator device 50 mayinclude depositing the acoustic reflector 54 over the substrate 52,followed by deposition of the bottom side electrode 60, followed bygrowth (e.g., via sputtering or other appropriate methods) of thepiezoelectric material bulk layer 64, followed by deposition of the topside electrode 68.

FIG. 7 is a schematic cross-sectional view of a film bulk acoustic waveresonator (FBAR) device 72 according to one embodiment. The FBAR device72 includes a substrate 74 (e.g., silicon or another semiconductormaterial) defining a cavity 76 that is covered by a support layer 78(e.g., silicon dioxide). A bottom side electrode 80 is arranged over aportion of the support layer 78, with the bottom side electrode 80 andthe support layer 78. A piezoelectric material bulk layer 84 embodyinginclined c-axis hexagonal crystal structure piezoelectric material(e.g., AlN or ZnO) is arranged over the bottom side electrode 80, and atop side electrode 88 is arranged over at least a portion of a topsurface 86 of the piezoelectric material bulk layer 84. A portion of thepiezoelectric material bulk layer 84 arranged between the top sideelectrode 88 and the bottom side electrode 80 embodies an active region90 of the FBAR device 72. The active region 90 is arranged over andregistered with the cavity 76 disposed below the support layer 78. Thecavity 76 serves to confine acoustic waves induced in the active region90 by preventing dissipation of acoustic energy into the substrate 74,since acoustic waves do not efficiently propagate across the cavity 76.In this respect, the cavity 76 provides an alternative to the acousticreflector 54 illustrated in FIGS. 6 and 7. Although the cavity 76 shownin FIG. 7 is bounded from below by a thinned portion of the substrate74, in alternative embodiments at least a portion of the cavity 76extends through an entire thickness of the substrate 74. Steps forforming the FBAR device 72 may include defining the cavity 76 in thesubstrate 74, filling the cavity 76 with a sacrificial material (notshown) optionally followed by planarization of the sacrificial material,depositing the support layer 78 over the substrate 74 and thesacrificial material, removing the sacrificial material (e.g., byflowing an etchant through vertical openings defined in the substrate 74or the support layer 78, or lateral edges of the substrate 74),depositing the bottom side electrode 80 over the support layer 78,growing (e.g., via sputtering or other appropriate methods) thepiezoelectric material bulk layer 84, and depositing the top sideelectrode 88.

In certain embodiments, an acoustic reflector structure is arrangedbetween the substrate and the at least one first electrode structure toprovide a solidly mounted bulk acoustic resonator device. Optionally, abackside of the substrate may include a roughened surface configured toreduce or eliminate backside acoustic reflection. In other embodiments,the substrate defines a recess, a support layer is arranged over therecess, and the support layer is arranged between the substrate and atleast a portion of the at least one first electrode structure, toprovide a film bulk acoustic wave resonator structure.

From the above disclosure of the general principles of the presentinvention and the preceding detailed description, those skilled in thisart will readily comprehend the various modifications, re-arrangementsand substitutions to which the present invention is susceptible, as wellas the various advantages and benefits the present invention mayprovide. Therefore, the scope of the invention should be limited only bythe following claims and equivalents thereof. In addition, it isunderstood to be within the scope of the present invention that thedisclosed and claimed articles and methods may be useful in applicationsother than surgical procedures. Therefore, the scope of the inventionmay be broadened to include the use of the claimed and disclosed methodsfor such other applications.

Example

BAW wafers (samples) and a blanket film were prepared according tomethods of the present disclosure and compared to a baseline BAW wafer(comparative sample) and blanket film prepared according to prior artmethods.

All samples, including the comparative sample, were prepared using adeposition system as described in U.S. patent application Ser. No.15/293,063 entitled “Deposition System for Growth of Inclined C-AxisPiezoelectric Material Structures.”

Three sample wafers (150 mm diameter) and a blanket film were preparedby depositing an AlN crystalline bulk layer in two steps directly onto asubstrate. The sample wafers were prepared by depositing a first portionof the AlN crystalline bulk layer in a first step at a deposition angleof 43 degrees and a second portion in a second step at a depositionangle of 0 degrees (normal incidence).

During the first step (at 43 degrees), the deposition pressure wasselected at 2.5 mTorr with an argon-to-nitrogen ratio of 6:15. The powerwas 3 kW, DC current was pulsed at 250 kHz, and the target voltage was250 V and target current 12 A. The distance from the target to thesubstrate was 100 mm. The deposition rate was 40 Å/min. The space wasnot heated but due to the use of plasma, the temperature was estimatedto be about 100° C.

During the second step (at normal incidence), the wafer substrate wasplaced on a stationary pedestal. The deposition pressure was selected at3.0 mTorr with an argon-to-nitrogen ratio of 1:5. The power was 6 kW, DCcurrent was pulsed at 100 kHz, and the target voltage was 250 V andtarget current 12 A. The space was heated to 300° C. A 100 W bias wasapplied to the pedestal. No collimator was used. The distance from thetarget to the substrate was 50 mm. The deposition rate was 900 Å/min.

A comparative (baseline) sample (150 mm diameter) was prepared bydepositing an AlN layer in one step. The AlN layer was deposited at adeposition angle of 43 degrees. The deposition pressure was selected at2.5 mTorr with an argon-to-nitrogen ratio of 6:15. The power was 3 kW,DC current was pulsed at 250 kHz, and the target voltage was 250 V andtarget current 12 A. The deposition rate was 40 Å/min. The space was notheated but due to the use of plasma, the temperature was estimated to beabout 100° C.

The blanket films were prepared to enable X-ray diffraction measurementof the c-axis angle of the AlN bulk-layer crystals. The AlN bulk-layercrystals deposited on the blanket films correspond to the AlN bulk-layercrystals deposited on the wafers under the same conditions.

The c-axis angle of the AlN bulk-layer crystals on the sample blanketfilm and the comparative blanket film was measured by a standard X-Raydiffractometer equipped with a goniometer for pole figure measurements.It should be noted that the deposition angles given in this example arenominal settings of the deposition system and some variation may beexperienced in the actual the range of angles at which the depositionflux contacts the substrate. However, the relative magnitude of theangles can still be compared.

The results are graphically shown in FIGS. 8A and 8B for the sampleblanket film, and in FIGS. 9A and 9B for the comparative (baseline)blanket film.

It was observed that by applying a major portion of the bulk layer atnormal incidence, a much higher than usual throughput could be achieved.The normal incidence deposition could be done without a collimator at900 Å/min. This deposition rate results in significant increases inthroughput and efficiency.

The samples prepared by the two-step method with the second step atnormal incidence had a c-axis tilt angle of approximately 35 degrees,similar to the comparative sample, as seen in the pole FIGS. 8A and 9A,respectively. It was observed that the bulk layer crystallites depositedat normal incidence aligned with the crystallites applied in the firststep. It was hypothesized that the crystallites of the first step act asa template for the subsequent bulk layer.

The effective electromechanical coupling coefficient and mechanicalquality factor of each wafer were evaluated by investigating thescattering (S−) parameter matrices of the samples using a vector networkanalyzer to extract resonator performance characteristics. Electricalprobing was performed across 100 locations on each wafer, and theresults were calculated as normalized averages.

Methods for computing quality factor (Q) and effective couplingcoefficient (k² _(eff)) were based on work published by K. M. Lakin,“Modeling of Thin Film Resonators and Filters” IEEE MTT-S MicrowaveSymposium Digest, 1992 pp. 149-152. Quality factor is determinedutilizing the following expression:

$Q = {\frac{1}{2} \times {frequency} \times \frac{dZphase}{dfrequency}}$

Effective coupling coefficient is determined by measuring series (f_(s))and parallel (f_(p)) resonant frequencies and utilizing the followingformula:

$k_{eff}^{2} = {{\frac{\phi \; s}{\tan \mspace{11mu} \phi \; s}\mspace{14mu} {where}\mspace{14mu} \phi \; s} = {\frac{\pi}{2}\left( \frac{fs}{fp} \right)}}$

The results are graphically shown in FIG. 10 showing electromechanicalcoupling coefficient (k_(e))² and in FIG. 11 showing mechanical qualityfactor (Q) normalized to the comparative (baseline) sample.

It was observed that the electrical performances of sample films grownunder deposition conditions according to the present disclosure werecomparable to the comparative (baseline) sample. The mechanical qualityfactor (Q) measured on the sample wafers was comparable or slightlyhigher (approximately 1.1 times) than the comparative (baseline) sample.

The results demonstrate that a bulk layer can be deposited in two stepsusing the methods of the present disclosure, where in the second stepthe deposition is done at a normal incidence angle, resulting in a bulklayer that is at least comparable or better than a bulk layer depositedusing prior art methods.

1. A method for preparing a crystalline bulk layer having a c-axis tilt,the method comprising: depositing a first portion of a bulk materiallayer onto a substrate at a first incidence angle, the first portionhaving a first c-axis tilt; depositing a second portion of the bulkmaterial layer onto the first portion at a second incidence angle thatis smaller than the first incidence angle, the second portion having asecond c-axis tilt that substantially aligns with the first c-axis tilt.2. The method of claim 1, wherein the first portion, the second portion,or both are deposited under deposition conditions comprising a pressureof less than 5 mTorr.
 3. The method of claim 1, wherein the pressure isfrom about 1 mTorr to about 4 mTorr.
 4. The method of claim 1, whereinthe first incidence angle is about 35 degrees to about 85 degrees. 5.The method of claim 1, wherein the first c-axis tilt is about 10 degreesto about 85 degrees.
 6. The method of claim 1, wherein the first c-axistilt is about 35 degrees to about 80 degrees.
 7. The method of claim 1,wherein the first c-axis tilt is about 35 degrees to about 52 degrees.8. The method of claim 1, wherein the first c-axis tilt is about 40degrees to about 50 degrees.
 9. The method of claim 1, wherein thecrystalline bulk layer exhibits a ratio of shear coupling tolongitudinal coupling of 1.25 or greater during excitation.
 10. Themethod of claim 1 further comprising depositing a seed layer prior todeposition of the first portion of the bulk material layer.
 11. Themethod of claim 1, wherein the second incidence angle is 0 degrees orgreater and up to 35 degrees.
 12. The method of claim 1, wherein thebulk material layer has a thickness of about 1,000 Angstroms to about30,000 Angstroms.
 13. The method of claim 12, wherein the thickness isabout 3,000 Angstroms or greater.
 14. The method of claim 1, wherein thefirst portion comprises AlN.
 15. The method of claim 1, wherein thesecond portion comprises AlN.
 16. The method of claim 1, wherein thesubstrate comprises a seed layer on which the first portion of the bulkmaterial layer is deposited.
 17. A structure comprising: a substratecomprising a wafer or a portion thereof; a piezoelectric bulk materiallayer comprising a first portion deposited onto the substrate and asecond portion deposited onto the first portion, the bulk material layerhaving a c-axis tilt of about 32 degrees or greater.
 18. The structureof claim 17, wherein the structure exhibits a ratio of shear coupling tolongitudinal coupling of 1.25 or greater during excitation.
 19. Thestructure of claim 17, wherein crystals in the bulk material layer aresubstantially parallel to one another and are substantially oriented ina direction of a preselected c-axis tilt angle.
 20. The structure ofclaim 17, wherein the piezoelectric bulk material layer comprises AlN.21. A bulk acoustic wave resonator comprising the structure of claim 17,and further comprising a first electrode and a second electrode, whereinat least a portion of piezoelectric bulk material layer is between thefirst electrode and the second electrode.